Field of the Invention
This invention relates generally to the field of wireless communication. In particular, the invention relates to antennas and beam steering techniques capable of multi-frequency band operation and adapted for use in wireless communications.
Description of the Related Art
Current and future communication systems will require higher performance from the antenna systems. As new generations of handsets, gateways, and other wireless communication devices become embedded with more applications and the need for bandwidth becomes greater, new antenna systems will be required to optimize link quality over larger bandwidths. Specifically, better control of the radiated field from the antenna system will be required to provide better communication link quality for an antenna system tasked to cover a wide frequency range.
Antenna diversity systems are often used to improve the quality and reliability of a wireless communication link. In many instances, the line of sight between a transmitter and receiver becomes blocked or shadowed with obstacles such as walls and other objects. Each signal bounce may introduce phase shifts, time delays, attenuations, and distortions which ultimately interfere at the receiving antenna. Thus, destructive interference in the wireless link is often problematic and results in a reduction in device performance. Antenna diversity schemes can mitigate interference from multipath environments by providing multiple signal perspectives. Antenna diversity can be implemented generally in several forms, including: spatial diversity, pattern diversity and polarization diversity. Spatial diversity for reception generally includes multiple antennas having similar characteristics, which are physically spaced apart from one another. Pattern diversity generally includes two or more co-located antennas with distinct radiation patterns. This technique utilizes antennas that generate directive beams and are usually separated by a short distance. Polarization diversity generally includes paired antennas with orthogonal polarizations. Reflected signals can undergo polarization changes depending on the medium through which they are traveling. By pairing two complimentary polarizations, this scheme can immunize a system from polarization mismatches that would otherwise cause signal fade.
Commonly owned U.S. Pat. No. 7,911,402, issued Mar. 22, 2011, and titled “ANTENNA AND METHOD FOR STEERING ANTENNA BEAM DIRECTION”, describes a beam steering technique wherein a single antenna is capable of generating multiple radiating modes; the contents of which are hereby incorporated by reference. In sum, this beam steering technique is effectuated with the use of a driven antenna and one or more offset parasitic elements that alter the current distribution on the driven antenna as the reactive load on the parasitic is varied. Multiple modes are generated, and thus this technique can be referred to as a “modal antenna technique”, and an antenna configured to alter radiating modes in this fashion can be referred to as an “active multimode antenna” or “active modal antenna”. An early application identified for this technique is a novel receive diversity application, wherein a single modal antenna can be configured to generate multiple radiating modes to provide a form of switched diversity. The benefits of this technique are the reduced volume required in the mobile device for a single antenna instead of a two antenna receive diversity scheme, reduction in receive ports on the transceiver from two to one, and the resultant reduction in current consumption from this reduction in receive ports.
An expansion of the switched diversity technique using a Modal antenna is to implement a two antenna receive diversity scheme such as an Maximum Ratio Combining (MRC) technique where one or both of the antennas are a Modal antenna. The additional radiation modes which result in additional radiation patterns generated by each Modal antenna will result in improved diversity gain.
Commonly owned U.S. Ser. No. 13/674,115, filed Nov. 12, 2012, and U.S. Ser. No. 13/749,627, filed Jan. 24, 3013, each describe a modal antenna or antennas used in a Multiple Input Multiple Output (MIMO) system to provide the capability to change the correlation coefficient between the pair of antennas dynamically. With 4G MIMO systems becoming more prevalent in the cellular communication field, the need for two or more antennas collocated in a mobile device are becoming more common. These groups of antennas in a MIMO system need to have high, and preferably, equal efficiencies along with high isolation and low correlation. For handheld mobile devices the problem is exacerbated by antenna detuning caused by the multiple use cases of a device: hand loading of the cell phone, cell phone placed to user's head, cell phone placed on metal surface, etc. For cell phone applications, the multipath environment is constantly changing, which impacts throughput performance of the communication link. A Modal antenna will provide the capability to compensate or alter the performance of the MIMO antenna pair as the environment changes.
As cellular networks become more heavily used and impacted by high data rates being accessed by a large number of users concurrently, a Modal antenna capable of generating multiple radiation patterns will provide system level improvements on both the transmit and receive function on mobile devices. A need for the Modal antenna capabilities at both transmit and receive frequency bands will complicate the design of the antenna system and will require attention be paid to the bandwidth that can be achieved for good correlation coefficient between the modes generated by the Modal antenna.
For a Modal antenna to provide improved communication system performance for either 3G or 4G systems it is important to provide an antenna capable of Modal operation at several frequency bands. U.S. patent application Ser. No. 13/227,361, titled “MODAL ANTENNA WITH CORRELATION MANAGEMENT FOR DIVERSITY APPLICATIONS” describes an antenna wherein multiple parasitic elements are proposed to provide the Modal antenna capability at several frequency bands. This technique provides the ability to optimize a parasitic element for a specific frequency band, with an additional parasitic element added to the antenna system when a second frequency band is required to possess the Modal antenna feature. This straight forward technique of adding or assigning a specific parasitic element for each frequency band required works well but requires additional volume for the additional parasitic elements and additional switches and reactive components to affect the loading of the parasitic elements. A method of providing the Modal antenna function at several frequency bands with a limited (ideally one) parasitic element is needed, along with a technique to dynamically alter the correlation coefficient bandwidth, to provide the ability to provide the Modal antenna function at transmit and/or receive frequency bands when desired without disturbing the performance of other frequency bands. Techniques and methods to address these issues are described in this patent.